Investment casting is an evolution of the lost-wax process whereby a component of the size and shape required in metal is manufactured using wax injection moulding. The moulded wax pattern is then dipped in ceramic slurry to create a shell; the wax is then removed and the ceramic shell fired to harden it. The resultant shell has open cavities into which molten metal can be poured to produce a metal component of the required shape and size. For example (but without limitation) the process is known to be used in the manufacture of turbine blades for gas turbine engines.
In gas turbine engines, the blades operate in an extremely high temperature environment. It is known to provide cooling channels within the blades through which cooling air can be circulated. These channels are known to be made by placing ceramic cores within a ceramic shell prior to casting the metal blade. The core has a geometry which defines the shape of the cooling channels within the resulting hollow blade. After the metal blade has been cast, the core may be leached from the cast blade, for example by use of an alkaline solution, leaving the hollow metal component.
Ceramic cores are known to be manufactured by particle injection moulding (PIM). In such a process, a ceramic material such as silica is suspended in an organic binder (also known as the “vehicle”) to create a feedstock. The feedstock is injected into a die cavity of the required size and shape to create a “green” component which comprises the ceramic and a binder. The binder is then thermally or chemically removed from the green component and the ceramic consolidated by sintering at elevated temperatures to provide the final ceramic core.
FIGS. 1 to 4 illustrate a typical die used for the PIM of a ceramic core for use in the investment casting of a turbine blade for a gas turbine engine.
FIG. 1 shows a ceramic core 1 for the formation of channels in the shape of an aerofoil. The core has a suction side surface 2, a pressure side surface 3, a leading edge surface 4, a trailing edge radius 5, a leading edge passage section 6, a trailing edge passage section 7, and an internal feature 8 separating the leading edge passage section 6 from the trailing edge passage section 7. The internal feature 8 typically maintains a gap separating the leading edge passage section 6 and trailing edge passage section 7 and may be in the form of a bump or rib extending from one of these sections and which maintains separation of these sections once the metal component has been cast and the ceramic core has been removed.
FIG. 2 shows a ceramic core die 9 that is used to form the core shown in FIG. 1. The core die 9 has a suction side forming half 10, and a pressure side forming half 11. It is known to have water cooling channels 12 that maintain the die at a constant and uniform temperature. Also illustrated are the elements 13 and 14 which form an internal feature within the core.
FIG. 3 shows a cut away view of the suction side forming core die half 10, showing a first cooling circuit 16 which provides temperature control to the leading edge forming part of the die and a second cooling circuit 17 which provides temperature control to the trailing edge forming part of the die. It is known to maintain the narrower trailing edge temperature slightly warmer than the wider leading edge temperature in order to help with the fill of the narrower regions of the component. It is also known to increase the temperature of injection, thereby allowing the ceramic paste to remain fluid for a greater period of time as it flows into the die. During cooling, the central portion of the ceramic core stays hot for a longer period of time. This can lead to surfaces of the internal feature 8 becoming significantly warmer than is desirable. Subsequently, this may lead to defects in that region which occur as the result of the ceramic core material adhering to the die as a consequence of the elevated local temperature.
The internal feature forming element 13, 14 may alternatively comprise a sacrificial insert manufactured separately from other parts of the core and die. An example is described in the Applicant's prior published U.S. Pat. No. 4,384,607. FIG. 4 is a reproduction of FIG. 2 of U.S. Pat. No. 4,384,607 and shows a sacrificial insert 22 used to form an internal feature within a core. In manufacture of the core, the insert 22 is located within a first pocket in one die half and a second pocket in the second die half. Ceramic core material is injected into the core die. The external core surface is formed by the die and the internal feature surface of the core is formed by the sacrificial insert. Following a prescribed cooling time, the ceramic core and insert assembly are removed from the core die. The insert may then be removed from the core, for example by dissolving in a solvent. The ceramic core is then fired in the standard manner. A problem identified with use of the sacrificial insert results from it typically being made from a material that has a lower coefficient of thermal conductivity than the core die. Consequently, the surface defects resulting on the surface of the internal feature 22 from local hot spots present due to ceramic core material cooling are generally more acute.
It is an object of the invention to provide an apparatus and method for providing a component by PIM wherein intricate features such as the internal feature described above can be formed with reduced surface defects.